Dovetail theorem: in triangle ABC, AD, BE and CF intersect at the same point O, and the area of triangle AOB is equal to BD and greater than CD; The area ratio of triangle AOB to triangle COB is equal to AE ratio Ce; The area of triangle BOC is equal to the area of BF ratio AF ratio triangle AOC; Therefore, the figure is named after the dovetail. It is one of the five models and a theorem about a plane triangle, commonly known as the dovetail theorem.